This invention relates to the production of aluminum chloride from clay. More particularly, this invention relates to an improved system for chlorination of clays in multiple stages in which the proportions and amounts of the chlorinating agent, reducing agent, alkali metal compound catalyst and silicon tetrachloride can be separately regulated to each stage to suit the needs of the aluminum chloride production reaction in each stage.
U.S. Pat. Nos. 4,086,320 and 4,096,234 describe the production of aluminum chloride by chlorination of clay containing aluminum oxide and silicon oxide. The reducing agent in such chlorination is a gaseous agent such as carbon monoxide gas or phosgene (COC1.sub.2), which can be used as both the reducing agent and the chlorinating agent. Chlorine gas can also be used as a chlorinating agent if carbon monoxide is used as the reducing agent. U.S. Pat. No. 4,096,234 further discloses the addition of a catalyst such as potassium aluminum chloride (KAlCl.sub.4) and recycling silicon tetrachloride back to the reaction zone to enhance the yield of aluminum chloride from the aluminum oxide present in clay while reducing the amount of silicon tetrachloride produced in the chlorination reaction. U.S. Pat. No. 4,086,320 discloses that the clay should preferably be comminuted to a particle size range of 90% by weight smaller than 0.6 millimeter to provide maximum recovery of aluminum chloride from the clay.
It is also known to use solid reducing agents in the chlorination of aluminous materials to form aluminum chloride. For example, U.S. Pat. No. 3,760,066 to Calcagno et al teaches the production of aluminum trichloride by reacting alumina particles with chlorine gas in a fluidized bed containing alumina and carbon particles, preferably coke, at 800.degree.-1000.degree. C.
Raw materials useful in the chlorination process are those containing an aluminum compound particularly where the major impurity is a silicon compound. Kaolin clays (such as kaolinite) are particularly well suited for the chlorination process.
It is desirable to operate a reactor for chlorination of clay on a continuous basis with a continuous flow of solid reactants through the reactor and a continuous flow of gaseous aluminum chloride from the reactor. This means that solid clay particles must be continuously fed to and discharged from the reactor and that the reaction bed consist of clay particles in various stages of aluminum depletion. The degree of aluminum depletion will depend on the kinetics of the reaction involved and on the residence time of the clay in the reaction zone.
Operation of a single stage reactor with a continuous flow of clay through it will result in discharge of a considerable amount of unreacted alumina unless the feed and discharge rates are quite low. However, low feed and discharge rates result in a low rate of conversion of the aluminous material into aluminum chloride.
The conflict between high production rates and good total extraction can be overcome by a process known as staging in which the solids are caused to follow a circuitous path through the reactor. Staging can be accomplished in a variety of ways as disclosed in the Chemical Engineer's Handbook by Perry and Chilton (5th Ed. 1973), page 20-64. As disclosed in such Handbook, the two basic types of staging are vertical and horizontal.
Vertically staged reactors involve countercurrent flow of gases and solids, which can be troublesome, especially when operated at elevated temperatures to process corrosive materials. For example, vertically staged reactors sometimes suffer from corrosion of the grids between stages, plugging of the grids, pressure drops between stages, gas leaks, and material handling problems. On the other hand, conventional horizontally staged reactors typically feed the same gas through all the stages and therefore aren't amenable to regulation of gas flow into the various stages for most efficient operation.